The invention relates to a fine grained sintered cemented carbide containing chromium, consisting of a first phase based on tungsten carbide (WC) which is bound by means of a second phase of a metallic binder based on Co or CoNiFe, and of at least one additional phase comprising at least one carbide or mixed carbide of tantalum.
Polyphase cemented carbides of this type have been known for a long time, the cemented carbide grades being distinguished according to the compromise between physical properties selected to vary depending on the particular intended use (hardness, bending strength, pressure resistance, modulus of elasticity). Accordingly, the cemented carbides are classified according to the international standard ISO/Tc29 into the P, M and K grades suitable for metal cutting purposes and the G grades preferably used for non-cutting shaping and parts subject to wear, which have different binder metal contents and different additional carbide contents, in addition to tungsten carbide as the main constituent. The harder a cemented carbide grade, the better its wear resistance, but the worse its bending strength.
In cemented carbides for metal cutting purposes, the quality of a cemented carbide grade is dictated quite substantially by its high-temperature properties. The hardness of the cemented carbides is in some cases reduced dramatically as temperature rises, while noxious scaling and diffusion processes increase at the same time, and the deformation behavior of cutting inserts or other bodies made from the cemented carbide likewise changes drastically.
A simple WC—Co cemented carbide having a cobalt content of about 10%, for example, will feature only about one third of its hardness at 800° C. compared with its hardness at room temperature, whereas a P10 cemented carbide containing additions of TiC and (Ta, Nb)C will still feature about half of its hardness compared with that at room temperature.
Finally, the mechanical properties of the sintered cemented carbides are affected by the way of their powder-metallurgical manufacture. Grain growth is almost impossible to avoid during sintering and hot isostatic pressing (HIPping) of the corresponding green compacts and has a negative effect on the bending strength of the sintered cemented carbide. Therefore, specific carbides are admixed to the starting powder blend as grain growth inhibitors. The most frequently used grain growth inhibitors are carbides of tantalum, chromium and vanadium, with tantalum carbide being usually employed as (Ta, Nb)C mixed carbide, owing to the natural association of the metals tantalum and niobium and for reasons of cost. The effectiveness of the three aforementioned commonly used grain growth inhibitors increases in the order mentioned, from Ta via Cr to V.
The complexity of the process sequences in cemented carbide manufacturing is still further increased in that both tungsten from the tungsten carbide and the metals of the grain growth inhibitors diffuse into the binder phase and dissolve therein to form a solid solution. Since the solubility of these metals in the binder metal is of course higher at higher temperatures than at room temperature, the solubility, that is, the maximum dissolved concentration at a particular temperature, can be exceeded, whereby the excessive amount which is no longer soluble precipitates out of the binder phase again or is deposited on the surface of the WC grains. Such deposition, however, impairs the wetting of the grains with the binder metal, which in turn results in a deterioration in the bending strength. A person of ordinary skill in the art thus has been and still is aware that only very small amounts of grain growth inhibitors should be utilized so as to achieve as good a compromise as possible between a prevention of grain growth during sintering and a deterioration of the wetting of the WC grains with binder metal.
Another technique at first suggesting itself for counteracting grain growth consisted in using finer starting powders from the outset, i.e. the use of fine (0.8 to 1.3 μm), submicron (0.5 to 0.8 μm), ultrafine (0.2 to 0.5 μm) or even nano particles (<0.2 μm), rather than coarse (2.5 to 6.0 μm) or medium coarse particles (1.3 to 2.5 μm), for the manufacture of the starting powder blend, even though the powders become more expensive as they increase in fineness, causing additional difficulties in handling and in producing the green compacts. Use of finer starting particles would obviate part of the grain growth inhibitors. This path was followed, in different ways, by U.S. Pat. No. 5,918,102 and German Patent 40 00 223.
According to U.S. Pat. No. 5,918,102 a powder blend of ultrafine WC and 6 to 15 wt. % binder metal is used, which is mixed with up to about 1 wt. % of grain growth inhibitors in the form of one or more of the carbides TiC, TaC, NbC, HfC, ZrC, Mo2C and VC.
German Patent 40 00 223 describes a microdrill made from a cemented carbide based on WC and Co, which is intended for producing submicron size holes in circuit boards and is manufactured from submicron tungsten carbide powder having an average particle size of 0.6 μm and having added thereto 4 to 10 wt. % (Cr+V), as related to the total weight of the binder alloy based on cobalt, because the authors of this printed publication have found that the grain growth of tungsten carbide can be more effectively prevented by the addition of such an amount of V and Cr than by an addition of tantalum carbide.
It turned out, however, that the cemented carbide grades mentioned in the two patents are not so suitable for metal cutting purposes, in particular for the cutting of steel, because they do not have the necessary high temperature properties (high temperature hardness, resistance to scaling and diffusion, wear characteristics in the cutting test).